do i need to run the aerator
 

If I have a sprinkler system as well as a cascading waterfall in my pond, is there a need for an aerator?

Hard to answer that question since there are lots of factors. I would say if you are happy with the water quality, you are not being plagued by algae growth and plants, fish and other aquatic life are happy the environment is getting sufficient aeration from the waterfall and I guess the sprinklers are keeping the water level up?

If parts of the waterscape are getting stinky or parts of it are stagnant and attracting insects, you probably need to add some airflow or other turbulence.

Do you test the water at all? Test results are within the limits you want? Then "if it ain't broke don't fix it!"